1. Field of the Invention
The invention generally relates to surface-emitting laser elements, surface-emitting laser arrays, optical scanner devices and image forming apparatuses, and more particularly to a surface-emitting laser element capable of emitting a laser beam in a direction perpendicular to a substrate, a surface-emitting laser array including therein a plurality of such surface-emitting laser elements, an optical scanner device having such a surface-emitting laser array, and an image forming apparatus having such an optical scanner device.
2. Description of the Related Art
Vertical cavity surface-emitting semiconductor lasers (i.e., VCSELs) are configured to emit a laser beam perpendicular to a substrate. The VCSELs have attracted much attention because of their low fabrication cost, their low power consumption, their compactability, their suitability for forming two-dimensional devices, and their high performance in comparison with edge-emitting semiconductor lasers that emit a laser beam in parallel with a substrate.
The VCSELs may be applied in the field of a light source for optical recording in a printer (oscillation wavelength: 780 nm), a light source for recording of an optical disk (oscillation wavelengths: 780 nm, 850 nm), and a light source for an optical transmission system such as Local Area Network or LAN utilizing optical fibers (oscillation wavelengths: 780 nm, 850 nm, 1.3 μm, and 1.5 μm). Further, the VCSELs may be applied as a light source for optical transmission between boards, within a board, between chips in a Large-scale integrated circuit or LSI, or within the chip of the integrated circuit.
In the above applied field of the VCSELs, light emitted from the VCSEL (hereinafter also simply called “emission light”) may preferably have high single transverse mode output power. Specifically, the VCSELs are frequently optimized for emitting light in high power basic transverse mode oscillation. Accordingly, the higher order transverse mode of the VCSELs may need to be controlled, and various attempts have been carried out to achieve this.
Japanese Patent Application Publication No. 2002-208755 (hereinafter referred to as “Patent Document 1”), for example, discloses a surface emitting semiconductor laser that includes a semiconductor substrate, a lower multilayer mirror, an active layer region and an upper multilayer mirror formed in this order on the substrate, an upper electrode having an opening bored for forming a laser beam emitting region, the upper electrode being formed on an upper layer of the upper multilayer mirror, and a current restriction part formed by insulating a periphery of a current channel and arranged between the upper electrode and the lower multilayer mirror. The aperture size of the opening of the upper electrode and the aperture size of the current restriction part are determined so as to increase the difference between the optical loss of a resonator in a higher-order transverse mode laser beam and the optical loss of the resonator in a basic transverse mode laser beam, based on the reflectivity of the resonator on a region corresponding to the upper electrode. The disclosed surface-emitting semiconductor laser causes a loss in a higher-order transverse mode laser beam exhibiting a high optical intensity in a region distant from a center of an emission region to selectively oscillate a basic transverse mode laser beam that exhibits the high optical intensity in the center of the emission region and low optical intensity in the region distant from the center of the emission region.
Specifically, the opening diameter of the electrode is 0.5 μm larger than the aperture size of the current restriction part.
Similarly, a surface emitting semiconductor laser disclosed, for example, in Japanese Patent Application Publication No. 2001-156395 (hereinafter referred to as “Patent Document 2”), U.S. Pat. No. 5,940,422 (hereinafter referred to as “Patent Document 3”), and Japanese Patent Application Publication No. 2006-210429 (hereinafter referred to as “Patent Document 4”) includes an upper electrode arranged to enclose a laser emitting surface, and a mode selection filter formed of a transparent dielectric film designed to provide reflectivity of a central portion of the laser emission region differing from that of the peripheral portion of the emission region. Specifically, in the disclosed surface-emitting semiconductor laser, the reflectivity of the central portion is configured to be higher than that of the peripheral portion within the emission region in order to facilitate a basic transverse mode operation. As a result, the surface emitting semiconductor laser is capable of outputting a high basic mode laser beam. Or in another disclosed surface-emitting semiconductor laser, the reflectivity of the peripheral portion is configured to be higher than that of the central portion within the emission region in order to facilitate a higher-order transverse mode operation.
However, in the surface-emitting semiconductor laser disclosed in Patent Document 1, since the optimal aperture size of the opening of the upper electrode and the optimal aperture size of the current restriction part are both narrow, it is difficult to mass produce the surface emitting semiconductor laser. In particular, when the surface-emitting semiconductor lasers are configured to form a surface emitting semiconductor laser array, it is difficult to achieve uniform emitting properties of the plural emitting portions of the surface-emitting semiconductor laser array. Further, since a periphery of an emission region is shielded from light, there is a large loss in the basic transverse mode laser beam exhibiting optical intensity that degrades the high output power of the laser beam.
In the surface emitting semiconductor lasers disclosed in Patent Documents 2 through 4, since the filter is transparent, there is little loss in the basic transverse mode laser beam in the periphery of an emission region. However, in these surface-emitting semiconductor lasers disclosed in Patent Documents 2 through 4, it is difficult to obtain stable high laser output power, and the surface emitting semiconductor lasers have a short life-span.
The above disadvantages of the surface-emitting semiconductor lasers disclosed in Patent Documents 2 through 4 are examined, and the factor causing the disadvantages may be specified as heat generated by high electric resistance (hereinafter called “element resistance”) of the surface-emitting semiconductor lasers.